At present the filter means are usually in the form of two flat grids, implemented in various ways and, in their in service position, closing off the sleeve transversely, together forming within the sleeve a dihedron with the line of intersection perpendicular to its axis and on the downstream side. In practise each grid pivots in the middle about an axis parallel to this line of intersection to occupy one or other of two other positions, namely a cleaning position which is the opposite of their in service position and in which they can be subjected to a washing counterflow, and a neutral position which is a position between the in service position and the cleaning position and in which they are in practise out of service, being aligned with the flow.
To enable the solid cleaning members to roll over them towards gathering means for these members along their line of intersection, the grids are necessarily at a relatively small inclination to the axis of the sleeve in its in service position.
The sleeve is therefore necessarily relatively long in order to accommodate them.
The length is in practise always greater than the diameter of the sleeve, on occasion 1.5 times the latter.
Installing a recovery device equipped with grids of this kind may be difficult, and even prohibitively uneconomic, in particular in some existing installations in which there is only a short length of pipe between the outlet from the heat exchanger and the masonry structure on which the latter rests.